Windows are extensively used in computers not only because they provide a user-friendly interface but also because they can be used in a multi-tasking environment. The processing of windows in computers was originally completely handled by the CPU. However, the visual performance of windows processed by the CPU is not very satisfactory. The visual quality of windows was improved by the advent of windows accelerators which implement the processing of windows operations.
The most common operation in a windows system is windows transfer, also called block transfer or BIT BLT. To accelerate block transfer in a windows system, several pixels can be transferred simultaneously rather than one at a time. In a 32-bit word access architecture or bus, where one pixel equal 8 bits, 4 pixels can be processed at one time. For instance, a 32 bit bus can move two words (1 pixel=8 bits; 1 word=16 bits) through it from a memory to a display. The speed is, thereby, trebled. However, during the process a word boundary problem occurs. This problem is illustrated in FIG. 3 and the following two examples. In these examples, 1 word=32 bits, 1 pixel=8 bits and 1 word=4 pixels.
In the word y: ##STR1## PO represents the position of the first pixel, P1 represents the position of the second pixel, P2 represents the position of the third pixel and P3 represents the fourth pixel.